Anthropic reports a context-window overflow with HTTP 400 and a message
of the form `prompt is too long: N tokens > M maximum`. The
`BAD_REQUEST` arm of `ProviderErrorCategory::from_http_status` only
recognized OpenAI's phrasings (`context_length_exceeded`, "exceeds the
context window"), and `parse_prompt_too_long` only ran for HTTP 413, so
these rejections were classified as `InvalidRequest` instead of
`PromptTooLarge`. This affects every wire path that classifies by
upstream HTTP status, including Zed cloud's `upstream_http_400` code,
and it silently disables any downstream handling keyed on the
`PromptTooLarge` category. This change sniffs the Anthropic phrasing on
400 responses too, carrying the provider-reported token count.

Closes AI-462

Release Notes:

- Fixed Anthropic "prompt is too long" errors reported with HTTP 400 not
being recognized as context-window overflows.